Bodies capable of conducting electricity, including bodies made entirely of metal and bodies having both metallic and nonmetallic portions, often have outer surfaces that need to be cleaned. Fabricated or machined metal products require cleaning, for example, prior to painting, coating, packaging or shipment. As another example, metal components which are to be remanufactured for the after-market almost always require some degree of cleaning. Similarly, bodies that are not capable of conducting electricity, that is, bodies that are non-metallic, often have outer surfaces that need to be cleaned.
Rust, scale, smut, petroleum derived contaminants, oils, greases, flux, carbonization, nonmetallic coatings, corrosion, paint, dirt and the like may form or be deposited on the surface of the body. These surface deposits or contaminants must be removed so that the body may be recycled and reused, or to prepare the body for subsequent surface treatment, while at the same time avoiding degradation of any non-metallic pieces, such as rubber and plastic, which may be present on the body to be cleaned. Metal cleaning, including precision cleaning and light/heavy industrial cleaning, is particularly important in industries which are involved in the forming, casting, extruding and machining of ferrous and non-ferrous metals. Examples of metallic bodies that may require cleaning include, for example, grocery carts, metallic brake shoes, jewelry, and electronic circuit boards.
Previously, cleaning of metals was typically accomplished using acidic cleaning solutions (having a pH of 6.0 or less). Prior to the recent increase in awareness of environmental concerns (such as handling safety, toxic exposure, disposal implications, etc.), acidic solutions were most frequently used because of their relative low cost, and their substantial effectiveness (both in total cleaning ability, cleaning speed and cost) in removing metal oxides, scale and other contaminants prior to pretreatment or painting. Typically, such solutions included mineral acids, chromic acid, carboxylic acids and other organic acids. Due to their very aggressive nature, the prior acidic solutions resulted not only in the removal of the undesirable contaminants on the item being treated, but often had the negative effect of potentially degrading the tank walls, pump components and other parts of the washer device itself Further, the solution often had to be replaced due to the change in the pH of the solution over time, and as a result, disposal of the spent solution (which solution today would almost certainly be classified as a hazardous substance, but at that time, most likely was not classified as such) was necessary.
One early method of cleaning metal bodies immerses the bodies in a high temperature cyanide bath. Major ingredients of the cyanide bath include caustic soda and sodium cyanide or potassium cyanide. The bath is heated to a temperature in excess of 700 degrees F. All of the cyanide and alkali materials are limited in life and have to be discarded and entirely fresh bath solutions made. The cyanide bath has the potential of liberating deadly cyanide gas, and the cyanide bath itself is a hazardous waste that requires special and expensive waste treatment and disposal.
More recently, with heightened public awareness and an increase in laws designed to protect the environment (as well as active governmental agencies to enforce such laws against industries, resulting not only in negative publicity to those industries, but also significant costs to accomplish the clean-up of existing waste sites, redesigning facilities, litigation, etc.), the metal cleaning industry began to utilize alkaline chemical solutions (having a pH of 8.0 or greater). These solutions typically use detergents and solvents, accompanied by high levels of agitation (such as by ultrasonic bath or high-pressure wash), to effect removal of contaminants. Alkaline cleaners have been formulated with such materials as sodium or potassium hydroxide, carbonate, bicarbonate, phosphate, silicate or other similar materials. The chemical reaction occurs via saponification with water-soluble soaps by neutralization of fatty acid soils. If the pH of the solution is kept between 8.0 and 13.0, these cleaners are somewhat successful in the removal of oils and greases. However, as with acidic solutions, the spent alkaline solutions must be frequently reprocessed, and further, they present similar hazardous waste disposal problems that previously were of no concern.
For example, to overcome the disadvantages of the cyanide bath, a variety of electrolytic cleaning systems were developed. Many of these systems use caustic soda (NaOH) to form a highly alkaline caustic soda bath. Caustic soda attacks galvanized steel, brass, bronze, copper, aluminum, magnesium, titanium and other metals. The caustic soda attacks the metal itself. Even if the metal could withstand immersion in the caustic soda bath, subsequent brushing or spraying treatments may be needed to remove tenacious impurities. The caustic soda bath is highly corrosive and requires special care in handling and disposal.
Neutral cleaning solutions (having a pH between 6.0 and 8.0) exist, and generally contain surfactants which act as wetting and emulsifying agents. With mechanical agitation or power washing, these solutions are useful for removal of oils, greases and other organic residues. However, care must given in selecting the appropriate solution (which can vary significantly depending on the contaminants being removed) and the types of mechanical agitation or power wash chosen for the process. Further, as with the acidic and alkaline cleaning solutions, reprocessing and/or replenishing of the solution is necessary, and special waste treatment and disposal procedures are necessary.
Other electrolytic cleaning systems have been proposed. One electrolytic system uses an electrolyte solution containing ferric sulfate and ammonium bifluoride. This system may generate objectionable fumes. The system does not de-scale or de-smut. A sludge containing insoluble salts of such metals as aluminum, copper, brass and bronze is generated that must be disposed of as a hazardous waste. Another electrolytic system uses an electrolyte solution composed of a phosphate alkaline material heated to 160-190 degrees F. Metallic ions, such as lead, tin, zinc or cadmium ions act as catalysts in the solution for removal of scale from stainless steel. These metallic ions remain in the spent electrolyte and require hazardous waste treatment and disposal. A film of metal may be deposited on the surface of the treated object. The film may be acceptable in cleaning stainless steel, but would be totally unacceptable in cleaning surfaces of other metallic objects such as circuit boards.
Yet another known method of clearing is directed to consumer cleaning of gold, silver, coins and jewelry. The object to be cleaned is immersed in an electrolyte and a relatively low voltage and amperage electric current is passed through the electrolyte solution. The method is designed specifically to remove tarnish, which are sulfides of gold and silver. During cleaning, hydrogen sulfide gas is created. Hydrogen sulfide is a noxious, poisonous gas that would present a serious problem in commercial operation.
The above conventional methods of cleaning metallic bodies require extremely high operating temperatures, toxic chemicals and/or highly corrosive liquids. These conventional methods are designed primarily to remove rust, scale or smut from iron or steel bodies, and are not suitable for cleaning of other types of metallic or non-metallic bodies. Further, the methods generate hazardous wastes that must be disposed of in compliance with environmental regulations and at high cost. Immersing the metallic body in the electrolyte may also be inefficient, as only a small number of bodies may be treated at a time.
Thus, there is a need for an improved method and apparatus for cleaning conductive and non-conductive bodies. This has become particularly critical in recent years, as a result of numerous restrictive environmental standards which have been mandated by local, state and federal governments. Most, if not all, of the existing processes and solutions used to clean contaminants and other materials from metal objects involve one or more of the following environmental considerations: health and safety concerns for persons handling the solutions and the waste products; formation of odors; foaming properties; environmental reporting requirements; waste treatment and waste disposal, and the costs and other difficulties associated therewith; and recycling costs and other difficulties associated therewith. The improved method should use non-toxic materials that are nonhazardous to personnel and should not require special disposal treatments or procedures.
Beyond the environmental issues which were not necessarily concerns when the existing processes were first developed, there are a number of practical shortcomings which are present in the known methods, including limited effectiveness in removing contaminants, short solution life, and the tedious and time-consuming task of altering key variables (such as range of agitation, range of chemical ingredients of the cleaning solution, time and temperature requirements, etc.) in order to determine the optimum level of each variable that will effectively and efficiently accomplish the desired level of cleaning. These variables may vary dramatically, depending on the composition of the body being treated, and on the particular contaminants for which removal is desired. Therefore, the improved method should clean a wide variety of bodies, and should not be limited to iron or steel. The improved method should be efficient and allow the cleaning of a large number of bodies at the same time. Treatment to clean the bodies should not harm the bodies themselves, but rather, should affect only the contaminants and other materials of which removal is desired.